As I understand how the rune MDT works, you get -15%MDT per rune harbored (against the element that rune is strong against), but this term is multiplicative with normal MDT, like Fan Dance gives -95%PDT~-20%PDT (depending on merits and gear) and is multiplicative with normal PDT. So a RUN with capped MDT (which is not at all hard to do), harboring two Lux and one Flabra, would take .5 * .7 = .35x normal Darkness damage and .5 * .85 = .425x normal Earth damage. The multiplicative nature of it is actually counterproductive. It's like with DA/TA/QA and OAx. Having a lot of both is good, but the more of one you have, the less effective the other becomes. 3 Runes without any MDT gives -45% MDT for that element. 3 Runes with capped MDT is -72.5%, which is relatively less ('cause that's how multiplicatives work, I hate math).

Point I'm trying to make is the very abilities that are supposed to make RUN a magic tank are in and of themselves crappier than a level 75 PLD shield, which can get you -75% MDT against all elements, instead of just one.

Magic Evasion is not enough. It would help. What Rune Fencer would need, if we want to keep in the theme of fencer and magic tanking, is have parry apply to spells and attacks that deal magic damage. Just have the RUN have a straight up chance to just completely deflect the spell. Not resist, a check before resist. I'm not exactly sure what all of the differences are between the magic resist checks and the accuracy equations, but just have a straight up level comparison + macc and parry comparison, and give RUN the ability to just completely parry magic attacks so they don't even get to the resist step. And allow it to work for everything.

Monsters with stupidly high macc will still be able to land spells, but a RUN with sufficiently high parry will still have a good chance to avoid them. It gives RUN a reason to go for more +parry, which a fencer would want, it makes it a better magic tank, which it needs to be, and it fits thematically. Whereas mere warriors might deflect an arrow off their shield, or monks block a blow, a rune fencer parries spells and curses and hexes, deflects them as others would normal slings and barbs and arrows.